The present invention relates to a method for controlling a hydraulic machine in a control system when utilizing the hydraulic machine as a pump, in which an electric machine is connected in a driving manner to the hydraulic machine. In particular, the invention relates to a method for limiting the pump pressure in a hydraulic system for a work machine.
The invention will be described below in connection with a work machine in the form of a wheel loader. This is a preferred but in no way limiting application of the invention. The invention can also be used for other types of work machines (or work vehicles), such as an excavator loader (backhoe) and excavating machine.
The invention will be described below in a control system which comprises a hydraulic machine which functions as both pump and motor. The hydraulic machine is connected in a driving manner to an electric machine which functions as both motor and generator. This type of control system is only to be regarded as an example and does not restrict the scope of the invention.
The hydraulic machine therefore functions as a pump in a first operating state and supplies pressurized hydraulic fluid to the hydraulic cylinder. The hydraulic machine also functions as a hydraulic motor in a second operating state and is driven by a hydraulic fluid flow from the hydraulic cylinder. The electric machine therefore functions as an electric motor in the first operating state and as a generator in the second operating state.
The first operating state corresponds to a work operation, such as lifting or tilting, being carried out with the hydraulic cylinder. Hydraulic fluid is therefore directed to the hydraulic cylinder for movement of the piston of the cylinder. On the other hand, the second operating state is an energy recovery state.
According to a previously known pump, there is a regulator in the pump that provides a pressure-limiting function so that the displacement of the pump is reduced in the event of too high a pressure.
A first object of the invention is to provide a control method that provides effective protection for the pump during operation.
According to an aspect of the present invention, a method is provided for controlling a hydraulic machine in a control system when utilizing the hydraulic machine as a pump, in which an electric machine is connected in a driving manner to the hydraulic machine, comprising the steps of detecting at least one operating parameter, of determining, on the basis of the detected operating parameter, whether a pressure supplied by the hydraulic machine is to be limited, and of controlling the electric machine correspondingly.
By this means, a pressure-limiting function is obtained. It is therefore possible to eliminate the pressure-limiting function incorporated in a conventional pump and thus to use a simpler/cheaper pump as the hydraulic machine.
According to a preferred embodiment, the method comprises the steps of determining a torque out from the electric machine on the basis of a torque in to the electric machine and the displacement of the hydraulic machine, and of controlling the electric machine correspondingly. The electric machine is also preferably controlled in a way that corresponds to the efficiency of the hydraulic machine. The torque out from the electric machine is suitably calculated on the basis of said input data.
According to a preferred example of the abovementioned method, the torque into the electric machine is detected. According to another example, various predetermined values of the torque in to the electric machine are utilized, preferably dependent upon the current work function.
Further preferred embodiments and advantages of the invention emerge from the following description.